1. Field of the Invention
This invention concerns a printed circuit board testing device having an adapter or a contact arrangement for electrically connecting the regular basic grid of an electronic testing device for single-layer or multi-layer, board-type wiring planes, such as printed circuit boards, ceramic substrates and the like, to the not necessarily grid-oriented contacts/connecting points of the wiring plane (test specimen) to be tested, which are in the form of metallic contact areas or connecting holes on one or both surfaces of the test specimen. In the test being discussed here, the wiring planes are not usually fitted with active electrical/electronic components, except where an IC or the like is being tested. In this connection it should be pointed out that the printed circuit board testing devices in question can be used for testing one side or both side of printed circuit boards.
2. Description of the Related Art
Many different versions of such contact arrangements (frequently referred to as "adapters" or "test jigs") are know inter alia from the literature "Pat. Abstract of Japan", Vol. 7, No. 278 (P-242) (1423), 10th December 1983 and JP-A-58 55 374 dated Sep. 16, 1983, or from IBM Technical Disclosure Bulletin, Vol. 24, No. 7A, December 1981, pages 3342 to 3344. Further such arrangements are know from GB-A-2 156 532, U.S. Pat. Nos. 4,099,120 and 4,443,756.
Furthermore, EP 184 619 B1 shows a printed circuit board testing device which has a basic contact grid with an "active basic grid" arranged upon it, which consists of spring test pins or contacts arranged in accordance with the basic contact grid in an insulating basic grid body, onto which a rigid adapter plate is laid, which compensates for local, increased connection densities on the test specimen and/or the lateral displacement (in the X and Y directions of this plate) between the regularly-arranged contacts of the basic grid and the not necessarily regularly-arranged contacts of the printed circuit board (test specimen) to be tested. This is achieved by arranging contact areas on one side of the adapter plate in accordance with the basic grid, whereas on the other side of this adapter plate the arrangement of contact areas assigned to the test specimen is identical to that of the test specimen. Printed conductors and plated-through holes on and in the plate, respectively, provide connection between the mutually-related contact area on each side of the adapter plate. In turn, vertically-arranged test probes, spring-loaded in their longitudinal direction, against whose points the contact areas of the test specimen are pressed by a suitable pressure-contact device, make contact with the contact areas of the adapter plate corresponding to the test specimen. Instead of such test probes, a further embodiment of this EP 184 619 B1 provides a "vertical conducting plate" that conducts current only perpendicularly to the plane of the plate. Both this elastically-compressible, vertical conducting plate and the spring test probes between the rigid adapter plate, which make contact with the test specimen, are necessary in order to produce the individual contact pressure required at each contact point for good contact.
In the first embodiment of this previously known arrangement, it is a disadvantage that, apart from the need for the adapter to be manufactured individually according to the usual printed circuit techniques anyway, a large number of spring test probes is also required, the provision of which represents a considerable investment for the user of the test equipment, since these test probes are required in substantially large numbers. In addition, due to the necessary mechanical strength of such spring test probes, limits are placed on their miniaturization and thus on the permissible density of the contacts or test probes.
These previously know problems are partially remedied in the above-mentioned further embodiment of EP 184 619 B1 in that the contact between the side of the adapter plate facing the test specimen and the test specimen itself is effected by the vertical conducting plate, which in principle consists of a mat of elastic, insulating material within which finely distributed contact wires run parallel to each other, perpendicular to the mat, and are thus insulated from each other, and which project at the top and bottom from the surface or the vertical conducting plate. The adapter plate and the test specimen can certainly be connected comparatively easily with the aid of such vertical conducting plates, but the manufacture of such vertical conducting plates is extremely expensive, and the use of such a contacting technique could be impeded by patents and the like in existence in various countries. Furthermore, these vertical conducting plates have only a limited life if they are continually and repeatedly loaded by the contact pressure required for making contact with the test specimens. Due to the continuously repeated bending load, particularly the ends of the contact wires projecting at the surface of the vertical conducting plate break off and gradually lead to the failure of the vertical conducting plate.
An adapter arrangement without test probes for testing devices of the type discussed here is shown in the applicant's EP 369 112 A1. Here, the necessary spring compliance for producing the required contact pressure at each test point of the test specimen is obtained in an embodiment in that a compressible plug of electrically-conductive elastomer is arranged on the essentially rigid adapter plate on each contact area. In a further embodiment described in this publication, this contact pressure is produced at each test point of the test specimen by constructing the adapter plate itself as a thin, flexible foil whose test specimen-specific contact areas are placed directly onto the test points of the test specimen. The side facing away from the test specimen is pressed in the direction of the test specimen by a spring contact arrangement.
This adapter arrangement, at least in the above-mentioned further embodiment, for the first time facilitates direct contact between the adapter foil and the test specimen by removing the means for producing the individual contact pressure at each test point of the test specimen from the area between the test specimen and the adapter plate or adapter foil, respectively. Since the conductive elastomer test probes or plugs with their limited miniaturization capability, or the known vertical conducting plate with its drawbacks as discussed, no longer have to be inserted in the area between the adapter plate or adapter foil, respectively, and the test specimen, after the adapter foil is placed with its test specimen-specific contact areas directly onto the test points of the printed circuit board to be tested, there are promising possibilities for increasing the density of test points which can be tested with an adapter arrangement. But in this adapter technique, which is based on direct contact between a flexible adapter foil and the test specimen in an adapter that is not hard-wired, practical difficulties are soon encountered with greatly increased miniaturization. This is especially true if, as in many current, conventional SMD wiring planes or printed circuit boards, the center-to-center distance between adjacent test points is only 0.2 mm and their area is approximately 0.1.times.0.5 mm. With such small test points and test point spacings on the test specimen or the test specimen-specific contact areas on the adapter foil, this can lead to contact faults which are entirely due to manufacturing length tolerances in the test specimens or the adapter foil.